Transpiles-to-(GNU-)C -- it was first written before LLVM, if we were starting the project today it would likely be a Clang fork.
I was curious about the implementation because I've had rough experiences with Vala and Nim's approach. Unlike with "transpiles-to-js" languages, transpiling to C has some tooling gaps (debugging being the big one). I admittedly don't have a ton of experience with either language but I couldn't find a plugin that gave me a step-through debugger for something like CLion or VS Code. You can debug the C output directly but this will turn off newcomers and assumes the C output is clean.
We intend to write a "proper compiler" at some point (probably either a Clang fork or a Cforall front-end on LLVM), but it hasn't been a priority for our limited engineering staff yet. I think we are getting a summer student to work on our debugging story (at least in GDB -- setting it up so it knows how to talk to our threading runtime and demangle our names), and improving our debugging capabilities has been a major focus of our pre-beta-release push.